The purpose of a chromatography system is to separate compounds of a sample with a chromatographic column. For example, a sample containing various compounds, dissolved in a solvent solution, may be injected into a mobile phase fluid stream with an injection valve, the fluid stream typically comprising one or more solvents. The sample-containing stream flows through the chromatographic column which may retain the compounds from the sample. The compounds from the sample experience a differential retention with the column's stationary phase, e.g., using packing material or sorbent within the chromatographic column, and the relative elution strength of the mobile phase. The separated compounds may then be directed to a detector for detection and analysis, where each of the compounds emerges from the chromatographic column at a different time corresponding to the respective differential retention of that compound within the chromatographic column. Detection over time results in “peaks” respectively corresponding to the components of the sample, where the magnitude of each peak correlates to the amount of the corresponding component in the sample. In preparative chromatography systems, the separated sample constituents may be collected by various fraction collection devices.
Typically, the sample-containing stream is a mixture of solvents provided by corresponding pump systems. The solvents include at least a strong solvent and a weak solvent referring to the solvents' relative elution strength in relation to each other and to the stationary phase being used. The strong solvent favors a partitioning of the sample components into the mobile phase, thus lessening retention, or providing faster transiting through the chromatographic column. The weak solvent favors partitioning of the sample components on the column's stationary phase thus increasing retention, and may serve to moderate the effects of the strong solvent. Attempts are made to balance the mobile phase composition or ratio between the strong and weak solvents in order to provide an acceptable compromise between speed of the chromatography operation and quality of the analytical results. However, when the sample is injected, the sample's own solvent adds to the partitioning effects of the mobile phase strong solvent. These partitioning effects favor residence in the mobile phase relative to the stationary phase and result in the sample smearing (not focusing) across a large portion on the column. This initial smearing caused by sample solvent effects represents a loss of resolution and broad signals seen with the detector. These degrading effects are even more pronounced in the large volume injections of preparative chromatography, and use in multi-dimensional chromatography where the sample solvent may represent a mobile phase segment cut from a separate chromatographic stream (one dimension) for injection into a second chromatographic stream (second dimension).
Two general types of chromatography systems are supercritical fluid chromatography (SFC) and high performance liquid chromatography (HPLC). SFC with packed columns typically uses an organic solvent, such as methanol, as the strong solvent and highly compressed dense carbon dioxide (CO2) as the weak solvent, SFC, the strong solvent may also be referred to as a modifier solvent. HPLC is generally practiced with two separation classes, normal phase and reverse phase. Normal phase HPLC may use a relatively polar solvent, such as isopropanol, as the strong solvent and a relatively non-polar solvent, such as hexane, as the weak solvent. Reverse phase HPLC may use an organic solvent, such as acetonitrile, as the strong solvent and water as the weak solvent. In other configurations of reverse phase HPLC, the organic strong solvent may also be referred to as a hydrophobic solvent or anon-polar solvent, and the aqueous weak solvent may be referred to as a hydrophilic solvent or a polar solvent.
An example of conventional SFC is provided by U.S. Pat. No. 6,576,125 to Berger et al. (issued Jun. 10, 2003), and an example of conventional HPLC is provided by U.S. Pat. No. 6,790,361 to Wheat et al. (issued Sep. 14, 2004), both of which are hereby incorporated by reference in their entireties. These patents are generally directed to standard injection techniques, in which the sample is injected into the full, combined mobile phase, and further include presenting the sample to the column in a diluted form at the same solvent composition as the mobile phase. However, such diluted sample loading is at the expense of increased loading times. Further, these techniques do not address samples dissolved in a sample solvent of greater solvent strength than the mobile phase strong solvent into which they are injected.
It is desirable to not only balance the mobile phase composition or ratio between the strong and weak solvents in the sample-containing stream entering the chromatographic column to optimize the speed and results of the chromatography operation, but additionally provide adjustability to the control relating to the dilution of the sample solvent with either or both components of the mobile phase and to the loading time of the sample as it is presented to the column. Such control would allow generally increased sample loading amounts and/or increased resolution in the separation of sample compounds. Further, such control would enable compensating for changes in solvent strength during the injection process, for example, when the sample solvents have greater strength than the mobile phase solvent. This requires precise, independent control of each solvent providing the sample-containing stream, both before and after injection of the sample, which is not present in the conventional chromatography systems.